The present invention relates to a method and device for converting the wavelength of light emitted from a semiconductor laser diode.
Crystals of non-linear optical materials such as lithium niobate (LiNbO.sub.3) are known to convert coherent light of one wavelength to light of another wavelength, such as the second-harmonic wavelength. Infrared light emitted by a semiconductor laser can be converted to blue light in this way. This capability has important implications for optical information storage, because the shorter wavelength of blue light increases the storage density, and for laser printing, where the shorter wavelength yields higher dot resolution.
Simply placing a bulk non-linear optical crystal in the path of a coherent beam does not lead to efficient wavelength conversion, however. Conversion efficiency depends on the distance traveled by the beam in the crystal (the longer, the better) and the beam intensity (the higher, the better). Even if coherent light is focused to a small, intense beam spot when it enters a crystal, diffraction quickly causes the beam to spread inside the crystal, so that the high intensity necessary for efficient conversion cannot be maintained for any appreciable distance.
The usual solution to this problem has been to fabricate a waveguide in the non-linear optical crystal. While propagating through the waveguide, the coherent beam maintains a substantially constant intensity, without spreading, and highly efficient conversion becomes possible. Many wavelength conversion devices of this type have been described. Besides having a waveguide, these devices often have a periodic domain inversion structure to correct the phase mismatch that occurs as wavelength conversion proceeds. Another common feature is a reflective structure that returns part of the incident light to the semiconductor laser as optical feedback, to control the lasing wavelength.
Wavelength conversion devices of the waveguide type are difficult to manufacture, however, because of the precise positional alignment needed to couple the laser beam into the waveguide.